1. Field of the Invention
The present invention generally relates to holograms and, more specifically, to improved reflection holograms which exhibit minimal chromatic dispersion.
2. Description of Related Art
High quality holographic optical elements are used in diffraction optic display systems, such as head-up displays (HUDs), for advanced aircraft, helmet-mounted displays, laser protective devices, narrow band reflective filters, and holographic high gain screens for simulators. These are only a few of the many uses of high quality reflective holograms.
Of particular relevance to this invention are displays consisting of an image source used to reflect light off of a mirror hologram to an observer. Such displays include head-up displays, helmet mirror displays, and certain types of holographic virtual image displays. In the reflection holograms of such displays, there may still be a need for adjusting peak wavelength, and for maximizing peak efficiency and spectral bandwidth of the hologram. Moreover, it may be desirable to use a broad band source for hologram playback and also have the ability to move the hologram image farther away from the hologram plane.
In reflection holograms created by a single exposure using conventional methods, the image diffraction angle .theta..sub.i, as defined in FIG. 1, is dependent upon wavelength .lambda. of the incident playback light and spatial frequency f=1/.LAMBDA. of the fringe pattern in the hologram. The image diffracted angle .theta..sub.i monotonically decreases with increasing wavelength, as is known. Furthermore, when illuminated by a beam having a range of wavelengths, the diffracted image rays cover a certain range of angles, which is sometimes hereinafter referred to as an image blur factor.
As can be appreciated, when it is desirable to increase the spectral bandwidth of the hologram, the image blur factor concurrently increases. Consequently the conventional method of making a reflection hologram has required a balancing between increasing spectral bandwidth and increasing image blur factor.
A need therefore exists in the art to provide an apparatus and method of making a reflection hologram that minimizes the image blur factor while maximizing the spectral bandwidth and, thus, maximizes the diffraction efficiency of the hologram.